There are two representative methods known in the prior art to develop latent images on the image forming material in electrophotography; one method uses a single-component developer which uses a magnetized toner that does not require a carrier, and the other method using a 2-component developer consisting of a non-magnetized or slightly magnetized toner and a magnetized carrier. The latter is considered to be advantageous in that it permits an easy control of the toner charged by friction, coloring toner freely as desired, and by its superior development characteristics. Thus, this method has been widely used. To improve the quality of the copied images, a method to develop an electrostatic latent image without directly rubbing the latent images by means of a magnetic brush formed of a 2-component developer, which is so-called non-contacting development method, has been proposed by this applicant (for example, in Japanese Patent O.P.I. Publication (Tokkai) No. 59-181362). The proposed method has another great advantage that it can be applied to a multi-color development system as disclosed in Japanese Patent O.P.I. Publication (Tokkai) No. 60-76766.
Further, the development of printers by the use of an electrophotographic process has been successfully growing. The printer of this type employs an image forming material with a sensitive layer to which an exposure by the use of laser or L.E.D. as its light source is given to form an electrostatic latent image which is subsequently developed with a toner consisting of charged particles. The image exposure is normally performed by scanning the image forming material with a light spot, however, in most coping operations a ground area or non-image portion is overwhelmingly larger than the colored area or image portion. Because of this, if exposure is performed in a manner in which light is irradiated to the ground area, the following problems may occur:
(i) The life of the light source will be shortened. PA1 (ii) The life of the image forming material will be shortened. PA1 (iii) The scanning unevenness of the optical system is likely to appear as lines on the ground area. PA1 1. Whenever the development of one color has been completed, the developed image needs to be transferred to the transfer material resulting in the need of a larger equipment and a longer image forming time. PA1 2. Repeated operations are liable to cause deviation from the original position. PA1 1 A uniform electrification is applied to an image forming material so that it can maintain a constant surface potential E. PA1 2 The first image exposure with such as laser, a cathode-rey tube, L.E.D. as its exposure source is given, and the potential of the exposure pH declines in proportion to its light amount. PA1 3 The electrostatic latent image thus formed is developed by a developing device to which a positive bias nearly equal to the surface potential E of the non-exposed portion is applied. As a result, the positively charged toner T.sub.1 adheres to the exposure portion pH with a relatively low potential thereby forming the first toner image. Although in the domain in which this toner image formed, the potential rises by DUP because the positive electrified toner T.sub.1 adheres to the domain, this potential normally does not become equal to that of the non-exposure region DA in strength. PA1 4 Next, the surface of the image forming material on which the first toner image was formed is electrified for the second time by means of an electrifier. As a result, a uniform surface potential E can be obtained regardless of the presence of toner T.sub.1. PA1 5 On the surface of this image forming material, the second image exposure is applied to form an electrostatic latent image. PA1 6 The development by means of a positively charged toner T2 with a color different from that of toner T1 is performed in the same manner as in the step 3 above, and thus the second toner image can be obtained.
To avoid these problems to take place, a method of forming a latent image by irradiating only the area to be colored and without irradiating the ground area have been widely used. In this case, different from normal electrophotographic copying operation, since the latent image formed has a lower electric potential in the image portion than that in the non-image portion and development must be performed by means of reverse development in which toner is adhered to the low electric potential portion of the latent image.
On the other hand, as means of multi-color image forming method, a variety of methods as described below has so far been proposed: According to one method heretofore known, a multi-color image formation is carried out by piling up different colored toner images on a recording sheet by repeating usual electrophotographic image forming process including electrification, exposure, development and transfer for different color toners. Namely, an electrostatic latent image is formed in accordance with each color information such as blue, green, and red, and the formed image is subsequently developed by means of such toners as yellow, magenta, cyan, or black-colored toners. Then the developed image is transferred to such transfer materials as a recording sheet or an overhead project film so that a multi color image can be formed on the transfer material by accomplishing the above process one by one for each color toner.
This method has, however, the following disadvantages:
Another multi-color image forming method has been proposed to solve the above disadvantages. In accordance with this method, plural piled up toner images are developed on an image forming material so that the transfer process can be completed at a time.
A variation of this technology that uses its desirable traits has also been proposed, wherein a multi-color image is formed by employing a means to fly toner particles to an electrostatic latent image formed on the image forming material while applying a bias containing a superimposed a.c. component to the developing device on and after the second development. In this method, no disturbance in the imposed colored toner image can occur as the developer layer does not rub the toner image formed in the previous stage.
Further explanation in regard to the performance of this multi-color image forming device is given with reference to the flow chart illustrated in FIG. 5 as follows. FIG. 9 shows the changes in the surface potential on the image forming material comprising a photo-sensitive material and a case when electrification polarity is positive has been taken to be an example; wherein PH is an exposed portion of the image forming material; DA, non-exposed portion of the image forming material; DUP, a rise in potential caused by the adhesion of the positively charged toner T1 to the exposed portion pH at the time of the first development.
Thereafter, similar process is repeated several times as required and a multi-color toner image is subsequently formed on the image forming material. This image is transferred to a transfer material, and a multi-color recorded pictorial image is obtained by fixing it by heating or applying pressure. In this case, the toner and electric charge that remain on the surface of the image forming material is cleaned and the material is then used for the next multi-color image formation.
In the method described in FIG. 9, at least the developing process described in step 6 must be performed so that the developer layer will not come into contact with the surface of the image forming material.
It should be noted that in the multi-color image forming method, a step to remove the electrification of the surface of the image forming material may be carried out before the commencement of each subsequent electrification. Also, either the same or different exposure source may be used for each image exposure step.
In the electrophotography, for example, a halogen lamp, gas or semiconductor, laser light L.E.D., CRT, or liquid crystal is used as a means for exposure.
As a means to form a latent image in multi-color image formation, besides the previously described electronic photography, a method of injecting electric charge directly to the surface of the image forming material by means of multi-needle electrodes or a method to form magnetic latent image by means of a magnetic head may also be used.
In the 2-component developer used for the developing method of this kind, to improve the resolution, tone reproductivity of the toner image and over all picture quality, attempts have been made to make the particle diameter of a carrier or toner as much small as is practically possible.
For example, Japanese Patent Applications Nos. 58-238296 and 59-22018 by the present applicant disclose a technology capable of performing a non-contacting development by the use of a carrier with a smaller particle diameter of less than 30 .mu.m instead of a conventional carrier with a larger particle diameter ranging from 50 .mu.m to 500 .mu.m, and a toner with a diameter of less than 15 .mu.m.
In this technology, however, if the carrier in the developer is made to have a smaller particle diameter, binding force of the carrier with the toner tends to be weakened. This may also cause contamination inside the device by the scattering of the carrier and toner while handling the developer or during the process of image formation. Further the carrier and the toner are likely to adhere to the surface of the image, thereby causing fog and this makes it difficult to obtain a clear toner image.
To remove such occurrence of fog, the distance between the image forming material and the developer carrier may be widened, however, this weakens the development electrode effect, making the development by the toner more difficult. The developing capability is improved by applying an electric field with an oscillating component between the image forming material and a developer transporting means, however, this often causes fog in the non-image portion and the scattering of the carrier, which makes it difficult in design to electrically isolate the developing device.
Generally, in the non-contacting developing method, the magnetic brush formed on the developer transporting means including a non-magnetic sleeve (hereafter often referred to as sleeve) is separated from the surface of the latent image, namely, the brush does not contact with the image. To have the toner scatter over the latent image, a voltage with an oscillating component, namely, an A.C. bias is applied to the sleeve. However, this A.C. bias causes the carrier to adhere to the image forming material. Especially, when trying to make a uniform height of the magnetic brush as small as possible the use of a carrier with a smaller particle diameter is advantageous, nonetheless, the binding force of the carrier to the sleeve is weakened, thereby causing the carrier to scatter easily inside the device.
In the reversal development, it is often difficult to make the potential level at the ground portion of the image forming material to be uniform. This is attributed to the microscopic fluctuations in the electric charge retaining ability in the photo-conductive layer, resulting in the difficulty to attain a uniform electrification on the surface of the image forming material. This means that microscopically different forces apply to the toner and carrier and, because of this, fog in the resulting image and carrier adhesion can easily occur. Especially these phenomena are more likely to occur when an organic photoconductive material.
To remove fog, the gap between the image forming material and the sleeve be widened, however, in this case, the previously described problems will need to be dealt with.
Further, when a developer containing a carrier with small particle diameter is used in a multi-color image forming method, the binding force of the sleeve with carrier and toner is weakened. This may cause the previously described scattering of carrier and toner inside the device leading to the contamination, fog on the resulting image due to the scattering of the toner and the carrier, which makes it difficult to obtain a clear cut image.
Although such fog may be reduced by widening the distance (hereafter referred to as an image gap) between the image forming material and the sleeve, this will weaken the electrode effect as mentioned hereinafter, making the development by the toner to become more difficult. If a large A.C. electric field is created between the image forming material and the sleeve, the developing capability may be improved whereas the fog caused by the toner and scattering of the carrier on the non-image portion may be aggravated, resulting in the difficulty in the design of the electrical isolation of the developing device. Further, different color toners may get mixed with one another in the developing device, causing image colors to become unbalanced.